I just learned that Xara displays the resolution of the bitmap in the page in the status bar:

Click image for larger version. 

Name:	resolution.png 
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Size:	24.5 KB 
ID:	104607

This information can be used to estimate the right resolution when creating bitmap copies or when exporting. With mold applied, the resolution is not displayed. To get to the unmolded image, right click and select Open mould.

What I forgot to mention earlier: To create a bitmap copy in place, shift click on Create.